Field of the Invention
The present invention concerns a method for controlling a magnetic resonance system that has multiple radio-frequency transmission channels via which, in operation, RF pulse trains (radio-frequency pulse trains) are emitted in parallel, each RF pulse train including at least one radio-frequency pulse. The RF pulse trains are initially determined so that a minimum B1 field maximum value is not exceeded by the radio-frequency pulse in the emission of the RF pulse trains. Moreover, the invention concerns a pulse optimization device in order to determine transmission scaling factors for the individual radio-frequency transmission channels within the framework of such an RF pulse optimization method, as well as a magnetic resonance system with such a pulse optimization device.
Description of the Prior Art
In a magnetic resonance system or magnetic resonance tomography system, the subject to be examined is typically exposed by means of a basic field magnet system, subject to a relatively high basic magnetic field (also designated as a “B0 field”) of 3 or 7 Tesla, for example, which aligns nuclear spins in the subject with the direction of the basic magnetic field. By means of a gradient system, a magnetic field gradient is additionally applied. By means a radio-frequency transmission system, radio-frequency excitation signals (RF signals) are then emitted by suitable antenna devices, in order to cause the nuclear spins of specific atoms in the subject to be excited to resonance. The radio-frequency field (also designated as a “B1 field”) has a defined flip angle relative to the magnetic field lines of the basic magnetic field that causes the nuclear spins to be deflected or “flipped” by the flip angle, with a spatial coding produced by the gradient field. This radio-frequency excitation or the resulting flip angle distribution is also designated in the following as a nuclear magnetization, or “magnetization” for short. The correlation between the magnetization m and the B1 field radiated over a time duration T is:
                    m        ≈                  2          ⁢                      πγ            ·                                          ∫                                  t                  =                  0                                T                            ⁢                                                                    B                    1                                    ⁡                                      (                    t                    )                                                  ⁢                                  ⅆ                  t                                                                                        (        1        )            wherein γ is the gyromagnetic ratio, t is the time variable, and B1(t) is the temporally variable magnetic field strength of the B1 field. Upon relaxation of the nuclear spins, radio-frequency signals (known as magnetic resonance signals) are radiated that are received by means of suitable reception antennas and then are processed further. Finally, the desired image data can be reconstructed from the raw data that is acquired in such a manner. The emission of the radio-frequency signals for nuclear spin magnetization usually takes place by means of a radiator known as a “whole-body coil” (also called a “body coil”), or sometimes with local coils placed on the patient or subject. A typical design of a whole-body coil is a cage antenna (birdcage antenna) that has multiple transmission rods that are arranged parallel to the longitudinal axis of the patient space in the system (scanner) and distributed around the patient space, in which a patient is located in the examination. The antenna rods are capacitively connected with one another in an annular form at their ends.
It has been typical to operate whole-body antennas in a “CP mode” (circularly polarized mode) or an “EP mode” (elliptically polarized mode). For this purpose, a single temporal RF signal is provided to all components of the transmission antenna, for example all transmission rods of a birdcage antenna. The transfer of the pulses with identical amplitude to the individual components typically takes place with a phase offset exhibiting a displacement or shift adapted to the geometry of the transmission coil. For example, for a birdcage antenna with 16 rods, the rods are respectively controlled with the same RF magnitude signal, offset with a 22.5° phase shift. The result is then a radio-frequency field that is circularly polarized in the x/y-plane, i.e. orthogonal to the longitudinal axis of the birdcage antenna running in the z-direction.
More modern systems already have multiple independent radio-frequency transmission channels via which different radio-frequency pulse trains can be emitted in parallel in operation. For example, in such systems the individual rods in a birdcage antenna are separately charged with radio-frequency pulses that are independent of one another. The emitted RF field then arises by a superposition of the signals emitted by the individual transmission channels.
It is thereby possible to precisely, individually establish the amplitudes and phases of the individual RF pulses in order to achieve a desired radio-frequency field pattern that is spatially shaped in a specific manner.
Moreover, the possibility exists to implement what is known as “RF shimming” or “B1 shimming”. In this method, only a single reference pulse train is initially provided, which is modified individually in amplitude and phase for every single transmission channel, such that ultimately a particularly homogenous B1 field is emitted in a defined volume under consideration. This B1 shimming takes place in a special adjustment step in which—as will be explained below—a suitable complex scaling factor is determined for each of the transmission channels, with which scaling factor the reference pulse train or its RF pulses in that channel are scaled.
In each, however, a complete pulse sequence is initially developed (designed) before the start of a measurement, and then, possibly after a B1 shimming, the measurement (data acquisition) is implemented wholly automatically using the predetermined pulse sequence, according to the requirements of a measurement protocol.
A problem with this conventional procedure is that, for technical reasons related to the apparatus, the components located in the transmission chain of the magnetic resonance system, such as RF amplifiers, cables, measurement apparatuses, adaptation networks, etc.—must be protected from overvoltages. Therefore, upon emission of a pulse train it is initially monitored as to its voltage compatibility with regard to the relevant components, and the voltage of the pulse train is thus limited. With the emergence of systems with two or more independent transmission channels, this voltage limitation (also generally designated as a B1 limit) can be limited differently by the phase relationship of the various channels. Since the RF pulses at the various channels not only have different pulse levels but also can be phase shifted counter to one another, voltage differences that are higher than the maximum amplitude of the individual RF pulses of the pulse trains can occur between these channels. This means that the B1 limit at the point in time of the adjustment of the protocol, or at the point in time of the design of the pulse sequence, is not yet known. However, a minimum duration of the RF pulses results from this limit due to the other predetermined boundary conditions for the creation of the pulse sequence (such as flip angle, bandwidth, pulse shape etc. of the RF pulses). The timing (the relative temporal arrangement of the pulses) of the entire pulse sequence is therefore hard-set and normally can no longer be changed without affecting important time parameters, for example the echo time (which is responsible for the contrast of the image data, among other things). The B1 limit in the design of the radio-frequency pulses thus does not coincide with the B1 limit calculated later during the adjustment or in the monitoring step, which can lead to inconsistencies that cause the pulse sequence not to be emitted as originally planned. This can lead to a reduced image quality.
In order to circumvent this problem, for example, the B1 limit could always be set to an absolute minimum B1 field maximum value. This would mean that, in the design of the pulse sequences, it is already ensured that the B1 limit would not be underrun even in the worst case. The timing within the pulse sequence then must accordingly always be selected so that the maximum necessary pulse length is assumed in order to achieve the desired flip angle with the allowed maximum amplitude of the radio-frequency pulses. However, if the pulse is more significantly extended than is actually necessary in order to maintain a defined amplitude, the bandwidth of the pulse is automatically strongly reduced in comparison to the optimal case. A reduced bandwidth of the radio-frequency pulses in turn increases the probability of artifacts due to chemical shift, or a greater susceptibility to artifacts of the B0 field.